Check valves used to stop the flow of fluid in one direction only and which automatically close when the direction of flow reverses are commonly made of metal and consist of a rigid gate which pivots about a point mounted in the valve housing. Generally such check valves automatically close using the gravity forces imposed upon the weighted pivotally mounted gate.
Such rigid, pivoting check valves must be periodically cleaned through "clean out" openings or they become inoperative due to the accumulation of solids carried in the fluid. Further, such metal valves corrode and require periodic maintenance to insure the proper operation of the bearings about which the gate pivots.
Even when properly maintained, a solid object carried by the fluid can become lodged between the gate and its seat thereby holding the valve open and permitting reverse flow of fluid. Metal check valves, in addition to being expensive, also can "chatter" if the fluid is subject to frequent reversals or surges in the fluid flow and cause a noise nuisance problem.
Standard check valves generally pass fluid proportional to the fluid pressure and thus in some applications will pass a steady flow and thus will not automatically flush but permit the buildup of solids carried by the fluid in and about the seat of the valve thereby interfering with the closing of the valve.
Metal rigid check valves are heavy and unless counterweighted, a relatively large pressure is required to permit the flow of fluid, especially in large diameter pipes. Standard check valves further require gravity for closure and therefore are not safely dependable in moving, tilting or inverted applications.